A number of solid hazardous and solid waste materials are produced by municipalities and industries each day. These include such diverse materials as municipal incinerator bottom ash, hazardous waste incinerator bottom ash, bag house or precipitator dust, steel plant dust, electroplating sludges, electrochemical machining sludges, waste foundry sands, dried and decontaminated sewage solids and coal combustion fly ash. In the past, such solid materials have been disposed of largely in landfills. Because of the potential scarcity of landfills, there is continual pressure to find other means for disposing of such wastes. Furthermore, many of the solid wastes contain one or more minor amounts of metals and metal oxides which are considered hazardous and thereby the whole of the solid waste is classified as hazardous.
Some of the waste materials contain appreciable amounts of valuable metals or oxides of such metals. These represent an important resource for the country if they can be recovered from the waste materials in a useful form.
For the foregoing reasons, there is a growing need to find useful applications for the waste streams. There is also a need to convert the hazardous materials into a form in which they are insoluble in ground water and pose no threat to the environment.
A number of workers have disclosed methods for disposing of waste containing radioactive substances. For example, U.S. Pat. Nos. 4,209,421 and 4,376,070 incorporate the radioactive waste in glass-forming material which forms a glass around the radioactive waste. In U.S. Pat. No. 4,395,367, a metal oxide, such as lead oxide, and a reducing agent are also mixed with the glass forming agent. After melting, the mixture separates into a glass phase containing the radioactive material and a metal phase obtained by reducing the metal oxide. The metal phase contains certain noble metals that can be recovered from the waste.
Two glass melters are described in Freeman, Innovative Thermal Hazardous Organic Waste Treatment Processes, pp. 44-54, Noyes Publications, Park Ridge, N.J. (1985). Waste streams are heated in a furnace containing molten glass. Noncombustibles mix with the glass which encapsulates the waste material when it solidifies.
A complex apparatus which combines oxidizers and other treatment chambers in series with a rotary kiln is disclosed in U.S. Pat. No. 4,922,841. This is said to be useful in treating hazardous waste in a combustion process. Some of the solid residue is recovered as aggregate said to be non-hazardous and some forms a clinker.
A recent patent, U.S. Pat. No. 4,793,933 issued to the assignee of the present application, discloses a method for treating metal hydroxide electroplating sludges. In this disclosure, the sludges are first converted to metal oxides by heating. The mixture is further heated with quantities of silica and soda to cause fusion of the mixture with formation of a slag having the metal oxides in chemical solution. Optionally, some of the metal oxides were reduced to free metal which separated from the slag.
We have now discovered that waste streams of diverse origins can be blended in special formulations that form a solution of oxides on heating. This solution, after cooling, forms a solid having a number of commercial uses. This process represents a distinct improvement over prior methods that require addition of extraneous glass or glass forming materials to the waste stream. Furthermore, the method provides for the recovery or recycling of various metals found in the waste streams.